1. Field of the Invention
The present invention relates to a two-wheel drive/four-wheel drive switching device for a vehicle.
2. Description of Background Art
A vehicle which is capable of running while switching between two-wheel drive and four-wheel drive has previously been known.
FIGS. 4 and 5 illustrate an example of such a vehicle. In both figures, the vehicle, indicated by reference numeral 1, is substantially composed of a body frame 3 with an engine 2 mounted centrally thereon. Front wheels 4 and rear wheels 5 are disposed on both sides of front and rear portions of the body frame 3. A steering handle 6 is disposed at a front upper position of the body frame 3 to steer the front wheels 4. A fuel tank 7 is mounted to the body frame 3 above the engine 2. Furthermore, a seat 8 is mounted behind the fuel tank 6.
As shown in FIG. 5, the front wheels 4 are supported vertically movably by suspension systems 9 disposed on both sides of the front portion of the body frame 3. Likewise, the rear wheels 5 are supported vertically movably by suspension systems 10 disposed on both sides of the rear portion of the body frame 3.
A final reduction gear 13 for the front wheels 4 and a final reduction gear 14 for the rear wheels 5, respectively, are mounted in front and rear central portions of the body frame 3. The final reduction gears 13 and 14 are connected to the engine 2 through propeller shafts 11 and 12, respectively. The right and left front wheels 4 and the right and left rear wheels 5 are connected respectively to the final reduction gear 13 for the front wheels 4 and the final reduction gear 14 for rear wheels 5.
For example, between the final reduction gear 13 for the front wheels 4 and the propeller shaft 11 is disposed a two-wheel drive/four-wheel drive switching device which makes and breaks the transfer of power to the front wheels 4, thereby switching between a rear-wheel drive mode and a four-wheel drive mode.
Alternatively, the two-wheel drive/four-wheel drive switching device is disposed between the rear propeller shaft 12 and the final reduction gear 14 for the rear wheels 5 to switch over between a front-wheel drive mode and a four-wheel drive mode.
This switching between a front-wheel drive mode and a four-wheel drive mode is conducted by the driver, as necessary, according to the state of the road surface and the running mode of the vehicle.
For example, a switching device of such a structure as shown in FIG. 6 has been proposed to be used as the two-wheel drive/four-wheel drive switching device of the background art.
Also in FIG. 6, a two-wheel drive/four-wheel drive switching device, indicated by reference numeral 15, is made up of two input shaft halves 16a and 16b. The input shaft halves 16a and 16b are formed by axially bisecting an input shaft 16 of the final reduction gear 13 for the front wheels 4 and butting the input shaft halves 16a and 16b in alignment with each other. Furthermore, a switching unit 17 is provided for connecting and disconnecting the input shaft halves 16a and 16b from and to each other.
To be more specific, a cylindrical positioning protrusion 18 is formed centrally on an end face of the input shaft half 16a, which is positioned on the final reduction gear 13 side. Furthermore, a positioning recess 19 for rotatably fitting with the positioning protrusion 18 is formed centrally on an end face of the input shaft half 16b, which is positioned remote from the final reduction gear side. Both input shaft halves 16a and 16b are in butting relationship with each other and the positioning protrusion 18 and the positioning recess 19 are fitted together, whereby both input shaft halves 16a and 16 are aligned and connected with each other so as to be relatively rotatable with respect to each other.
Outer peripheral surfaces of the abutting portions of both input shaft halves 16a and 16b are splined (not shown) and the switching unit 17 is disposed so as to surround the abutting portions.
The switching unit 17 has a splined inner surface and is slidably fitted to the abutting portions of both input shaft halves 16a and 16b. The switching unit 17 is made up of a switching ring 20 adapted to be engaged with and disengaged from each of the splines and a drive mechanism 21 constituted by a solenoid. The drive mechanism 21 causes the switching ring 20 to slide axially with respect to both input shaft halves 16a and 16b to selectively move to a position where the switching ring 20 is brought into engagement with the spline of only one input shaft halve 16a and a position where the switching ring is brought into engagement with the splines of both input shaft halves 16a and 16b. 
In the two-wheel drive/four-wheel drive switching device 15, the switching ring 20 is moved in one direction by the drive mechanism 21 and is brought into engagement with only one input shaft half 16a to cut off the transfer of driving force to the front wheels 4, affording a two-wheel drive mode of driving only the rear wheels. Furthermore, the switching ring 20, in an engaged state thereof with one input shaft half 16a, is slid into engagement with the other input shaft half 16b to connect both input shaft halves 16a and 16b with each other, thereby transmitting the driving force to the front wheels 4 and thus affording a four-wheel drive mode in which the front and rear wheels 4 and 5 are driven simultaneously.
In the two-wheel drive/four-wheel drive switching device 15 of the background art, there remains the following problems to be solved.
In the case where a braking operation is performed with the rear brake in a two-wheel drive mode, for example, a braking force is exerted on the rear wheels 5 and not on the front wheels 4, so that a difference in brake feeling between two- and four-wheel drive modes occurs. The same problem occurs under the action of engine braking as well.
For solving such a problem with the background art device, it is necessary to construct a brake setting which is consistent in both two-wheel drive and four-wheel drive modes. This can be done by using a complicated control mechanism, which, however, results in an increase of cost.
The present invention has been accomplished in view of the above-mentioned problems of the background art, and it is an object of the present invention to provide a two-wheel drive/four-wheel drive switching device capable of exerting both rear braking and engine braking on the front and rear wheels.
According to a first aspect of the present invention, there is provided a two-wheel drive/four-wheel drive switching device for a vehicle which is capable of running while switching between two-wheel drive and four-wheel drive, wherein when the rear wheel speed becomes lower than the front wheel speed in a two-wheel drive mode, switching is made to the four-wheel drive mode.
According to a second aspect of the present invention, there is provided a two-wheel drive/four-wheel drive switching device for a vehicle, wherein a switching unit is provided in a power transfer mechanism disposed between an engine and front wheels or between the engine and rear wheels. The switching unit functions to make and break the transfer of power in the power transfer mechanism, and comprises a drive shaft connected to a drive side, a driven shaft fitted to the drive shaft with an annular clearance therebetween, a plurality of engaging and disengaging members disposed within the clearance between the drive shaft and the driven shaft and brought into engagement with and disengagement from opposed surfaces of both drive and driven shafts to connect and disconnect the shafts from and to each other, a switching mechanism for positioning the engaging/disengaging members selectively at a connecting position and a disconnecting position for the drive shaft and the driven shaft, and an elastic member for biasing said engaging/disengaging member in the direction where the drive shaft is engaged to the driven shaft, wherein the engaging/disengaging members are moved in a direction to disconnect the drive shaft and the driven shaft from each other against the elastic force of the elastic member, with rotation of the drive shaft in a two-wheel drive mode.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.